Anh,
Sorry for the delay. I just FTP'd in and did a lot of debugging. All indications seem to be that the Secret Key for your FB Application is wrong or not functioning properly.
Can you go to the
Facebook App Developer area
and ensure the each character of your Secret Key has been properly entered into the Configuration -> Facebook area of JFBConnect?
If that seems correct, then please click the "(reset)" link next to the App Secret in the Application area of Facebook. Then, enter that new secret into JFBConnect and see if that helps.
Please let me know if the above helps, and if so, which one. We'd love to know what's going wrong.
Thanks,
Alex